xfs: protect xfs_sync_worker with s_umount semaphore

xfs_sync_worker checks the MS_ACTIVE flag in s_flags to avoid doing
work during mount and unmount.  This flag can be cleared by unmount
after the xfs_sync_worker checks it but before the work is completed.
The has caused crashes in the completion handler for the dummy
transaction commited by xfs_sync_worker:

PID: 27544  TASK: ffff88013544e040  CPU: 3   COMMAND: "kworker/3:0"
 #0 [ffff88016fdff930] machine_kexec at ffffffff810244e9
 #1 [ffff88016fdff9a0] crash_kexec at ffffffff8108d053
 #2 [ffff88016fdffa70] oops_end at ffffffff813ad1b8
 #3 [ffff88016fdffaa0] no_context at ffffffff8102bd48
 #4 [ffff88016fdffaf0] __bad_area_nosemaphore at ffffffff8102c04d
 #5 [ffff88016fdffb40] bad_area_nosemaphore at ffffffff8102c12e
 #6 [ffff88016fdffb50] do_page_fault at ffffffff813afaee
 #7 [ffff88016fdffc60] page_fault at ffffffff813ac635
    [exception RIP: xlog_get_lowest_lsn+0x30]
    RIP: ffffffffa04a9910  RSP: ffff88016fdffd10  RFLAGS: 00010246
    RAX: ffffc90014e48000  RBX: ffff88014d879980  RCX: ffff88014d879980
    RDX: ffff8802214ee4c0  RSI: 0000000000000000  RDI: 0000000000000000
    RBP: ffff88016fdffd10   R8: ffff88014d879a80   R9: 0000000000000000
    R10: 0000000000000001  R11: 0000000000000000  R12: ffff8802214ee400
    R13: ffff88014d879980  R14: 0000000000000000  R15: ffff88022fd96605
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #8 [ffff88016fdffd18] xlog_state_do_callback at ffffffffa04aa186 [xfs]
 #9 [ffff88016fdffd98] xlog_state_done_syncing at ffffffffa04aa568 [xfs]

Protect xfs_sync_worker by using the s_umount semaphore at the read
level to provide exclusion with unmount while work is progressing.
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_sync.c

@@ -386,11 +386,11 @@ xfs_sync_worker(
 	 * We shouldn't write/force the log if we are in the mount/unmount
 	 * process or on a read only filesystem. The workqueue still needs to be
 	 * active in both cases, however, because it is used for inode reclaim
-	 * during these times. hence use the MS_ACTIVE flag to avoid doing
-	 * anything in these periods.
+	 * during these times.  Use the s_umount semaphore to provide exclusion
+	 * with unmount.
 	 */
-	if (!(mp->m_super->s_flags & MS_ACTIVE) &&
-	    !(mp->m_flags & XFS_MOUNT_RDONLY)) {
+	if (down_read_trylock(&mp->m_super->s_umount)) {
+		if (!(mp->m_flags & XFS_MOUNT_RDONLY)) {
 			/* dgc: errors ignored here */
 			if (mp->m_super->s_frozen == SB_UNFROZEN &&
 			    xfs_log_need_covered(mp))
@@ -398,9 +398,12 @@ xfs_sync_worker(
 			else
 				xfs_log_force(mp, 0);
 
-		/* start pushing all the metadata that is currently dirty */
+			/* start pushing all the metadata that is currently
+			 * dirty */
 			xfs_ail_push_all(mp->m_ail);
 		}
+		up_read(&mp->m_super->s_umount);
+	}
 
 	/* queue us up again */
 	xfs_syncd_queue_sync(mp);